In order to miniaturize power supplies, the control for power supplies should be simplified. Switching frequencies have been increased to reduce capacitive and magnetic elements in miniaturized power supplies. Still, in today's and future power supplies there is a need to reduce the number of components needed to control the power supply. Among the greatest challenges is communication across the isolation boundary. Several techniques exist. These include opto-couplers, transformers, radio frequency devices, etc. This invention provides a method and circuit that use the same transformer that is used for power conversion, i.e. the power transformer. The technique of this invention is not to send complicated encoded information, but to actually change the supply's effective duty cycle from the secondary.
A preferred embodiment of this invention is a self-oscillating flyback converter. The method and circuitry of the invention can be used with other topologies as well. The flyback converter is believed the best topology to illustrate the invention and the most easily understood. Plus, this topology offers a reduced number of control parts, which illustrates the advantage of this invention and makes it a preferred embodiment.
FIG. 2A shows a typical self-oscillating flyback converter in which an input voltage source 16 supplies a primary winding 18 of a transformer 32 in series with a primary switch 8. Its turn OFF command is controlled by a combination of a transistor 22 and positive feedback from a winding 20, a capacitor 14 and a gate resistor 12. Turn ON is primarily controlled with the same winding circuit, plus a resistor 10 provides turn ON at initial startup. The transistor 22 is turned ON if the current through a resistor 24 is sufficiently high when the switch 8 is ON. During the times the switch 8 is OFF, it provides a voltage 25 alternating in polarity to the output circuit. When the output load circuit comprising a load 31 and a filtering capacitor 30 is coupled, during the OFF time of the switching period, current flows through secondary winding 26 of the transformer 32 and through a diode D1 to the output load 31 and the filter capacitor 30.
FIG. 2B displays certain voltage and current versus time waveforms for the self-oscillating flyback circuit parameters, i.e.: primary gate voltage 34 of the switch 8, primary drain voltage 36 of the switch 8, and the currents-transformer primary winding current 40 and the secondary winding current 38.
A first, main drawback of this approach is that there is no regulation. The control is simple, but adding regulation would ordinarily add complexity and parts. Plus, in the past, to regulate the output well, an opto-coupler was added. A second drawback is that for low output voltages the diode has a relatively large forward voltage drop. A third drawback is the primary switch 8 does not have zero voltage switching in all situations. By adding a synchronous rectifier in place of the diode D1, to serve as a switch in the secondary circuit, the second drawback can be avoided. However, this could introduce other problems including cross-conduction between the primary switch 8 and the synchronous rectifier which needs to be avoided.
There remains, therefore, a need for an approach to using primary and secondary switches in a converter such that self regulation is accomplished, the switch in the output secondary circuit exhibits a low forward voltage drop, zero voltage switching is accomplished and cross-conduction is avoided.